Method of operating a switched-mode power supply and switched-mode power supply

ABSTRACT

In the case of known switched-mode power supplies with integrated preconditioner, the control curves are largely U congroent, but deviate from one another in low-load operation, whereby the unregulated intermediate circuit voltage increases. To improve efficiency in low-load operation, closed-loop control of the burst cycle is effected on the primary side of the voltage rns- former. The intermediate circuit voltage is limited to a permissible limit value. The closed-loop control device in the switching stage taps the intermediate circuit voltage at a voltage divider which is arranged between the preconditioner functional unit and the switching stage. An assembly additionally monitors the output voltage, for example by means of an optocoupler. Burst mode corn- pnses one or more burst cycles. A burst cycle is started when the intermediate circuit voltage reaches its limit value. At this time, the voltage transformer is switched off. A burst cycle ends when the output voltage reaches a minimum value. At this time at the latest, the voltage transformer is switched on again. Burst mode is terminated again either when the switched-mode power supply is again operated under normal load or when a maximum time has elapsed, measured from the start of burst mode.

The invention relates to a method of operating a switched-mode powersupply, which comprises a rectifier and a voltage transformer withprimary and secondary side. As a rectifier it is possible to use, forexample, a separate or an integrated preconditioner, in particularactive AC/DC converters may be used as preconditioners. As a result ofregulations relating to mains pollution caused by electronic devices,active AC/DC converters are used ever more frequently. In presentswitched-mode power supplies with “MAINS HARMONIC REDUCTION”, separatepreconditioners are used, which are provided in a completely separatestage in the power supply. Although standards are thereby met andfrequently even surpassed, the costs are at the same time unusuallyhigh. In the case of other known switched-mode power supplies, a passivesolution comprising a coil with sufficiently high inductance is usedinstead of active AC/DC converters. However, this passive solutionbrings with it the disadvantages of larger volume, increased weight,higher costs and lower efficiency.

Moreover, switched-mode power supplies with integrated preconditioner(AC/DC converter) and voltage transformer (DC/DC converter) are known,which are operated with at least one transistor, a half bridgecomprising 2 transistors or a full bridge comprising 4 transistors andthus also with a closed-loop control. Through suitable topology andcircuit dimensioning, it is ensured that the control curves of thepreconditioners and of the voltage transformer of these knownswitched-mode power supplies with integrated preconditioner are largelycongruent, wherein the preconditioners fulfill the functions “Control ofinput current waveform” and “Control of intermediate circuit voltage”and the voltage transformer fulfills the function “Stabilization ofoutput voltage and output current”. However, the disadvantage is that inlow-load operation, i.e. in the case of a load below a given powerthreshold (dependent on the design of the voltage transformer andoptionally on the line voltage), the control curves cease to becongruent and the unregulated intermediate circuit voltage increases. Inthis case, the maximum permissible limits for the components areexceeded and the voltage transformer is destroyed or relatively highlosses occur.

A known method of improving efficiency in low-load operation of aswitched-mode power supply is the burst mode. In burst mode, theswitched-mode power supply is switched on and off at a frequency whichmay be fixed or variable. The power reduces within the ratio of the ontime to the sum of the on and off time [on/(on+off)]. In the case ofknown switched-mode power supplies, this burst mode is activated fromthe secondary side, for example in order to reduce consumption instand-by operation.

It is therefore an object of the invention to provide a method for open-and/or closed-loop control of the intermediate circuit voltage of aswitched-mode power supply with integrated preconditioner, which allowslow-load operation and at the same time achieves high efficiency in saidoperation. It is a further object of the invention to provide aswitched-mode power supply which exhibits high efficiency in low-loadoperation.

The object is achieved according to the invention, with regard to themethod, in that a limit value is fixed for the intermediate circuitvoltage. When the limit value is reached, a burst mode is activated.Control of the burst mode is effected on the primary side of the voltagetransformer. The intermediate circuit voltage is limited to a maximumpermissible limit value.

The object is achieved with regard to the switched-mode power supply bya switched-mode power supply comprising a preconditioner and a voltagetransformer, in which an intermediate circuit voltage is measured andwhich comprises means for

monitoring the intermediate circuit voltage

comparing the intermediate circuit voltage with a fixed limit value

activating a burst cycle when the limit value is reached on the primaryside of the voltage transformer.

In the case of a variant of the switched-mode power supply according tothe invention, the intermediate circuit voltage and theopen-/closed-loop control device of the switching stage have the sameground potential.

According to the invention, open- and closed-loop control of the burstmode is dependent on the intermediate circuit voltage. Since the open-and/or closed-loop control device and the intermediate circuit voltagehave the same ground potential, simple measurement is possible, e.g. bymeans of a voltage divider. An increase in this voltage shows thatlittle power is being consumed on the secondary side. Consequently, alimit value is fixed and monitored for the intermediate circuit voltage.When this limit value is reached, burst mode is activated on the primaryside of the switched-mode power supply. This means that theswitched-mode power supply is switched off, wherein the voltage in theintermediate circuit remains virtually constant, while the secondaryvoltages, i.e. the output voltages, fall slowly. After a previouslydetermined time or when a minimum value is reached for the secondaryvoltage, the power supply is switched on again. Due to the fall insecondary voltage, the intermediate circuit load is high for the firstmoment after switching on, while the intermediate circuit voltage falls.If the output load remains low, the intermediate circuit voltage risesagain after a short time and the burst mode is re-triggered as soon asthe intermediate circuit voltage reaches the limit value. If the outputload increases, the limit value for the maximum intermediate circuitvoltage is not reached and the burst mode is automatically deactivated.

Activation and deactivation of the burst mode is effected by anopen-/closed-loop control device, which is preferably arranged in aswitching stage. In a preferred embodiment of the invention, a singleopen-loop control is used, to measure and monitor the intermediatecircuit voltage and to activate and deactivate the burst mode.Activation of the burst mode means switching off the voltagetransformer.

In a variant of the switched-mode power supply according to theinvention, the voltage transformer is switched on again, when apreviously determined time has elapsed which is measured from the timeof the start of the burst cycle. This means that the voltage transformerremains switched off for a previously determined time, before it isswitched on again.

In another variant, the voltage transformer is switched on again, whenan adjustable or adaptive time has elapsed which is likewise measuredfrom the time of the start of the burst cycle. Once this adjustable oradaptive time has elapsed, the voltage transformer is switched on again.

In another variant according to the invention, the voltage transformeris switched on as a function of the input line voltage, e.g. at zerotransition. The input line voltage, for which a limit value exists, ismonitored by the closed-loop control. When this limit value is reached,the closed-loop control deactivates the burst mode and the voltagetransformer is thus switched on again.

In a further variant of the invention, switching on of the voltagetransformer depends on an additionally monitored secondary voltage, i.e.the output voltage of the switched-mode power supply. When the minimumvalue of the output voltage is reached, burst mode is deactivated by theopen-/closed-loop control and thus the voltage transformer is switchedon again.

The open-/closed-loop control device is preferably provided in aswitching stage which monitors and provides open- and/or closed-loopcontrol of both the intermediate circuit voltage and the output voltage.

In a variant of the method according to the invention, thepreconditioner functional unit and the switching stage are integrated.

The voltage transformer may be a DC/DC converter, for example, whichcomprises at least one transistor which is intermittently activated.

In a further variant of the invention, the burst mode is not activatedif a high output load is present at the same time. This means that, whenthe limit value Ulimit of the intermediate circuit voltage Uic isexceeded, the burst mode is not activated if at the same time the outputvoltage Uo is below the rated voltage Unorm. Such a (short-term)exceeding of the limit value of the intermediate circuit voltage Uic maybe triggered for example by a disturbance pulse in the power mains(surge).

In another variant, the load, which is connected on the output side tothe switched-mode power supply, may comprise a device for direct orindirect monitoring of the output voltage of the switched-mode powersupply and effect load variations as a function of the output voltage orthe activity of the switching stage. In this way, load variations onlytake place during active phases of the switching stage. Detection of theactive phases of the switching stage may occur:

through measurement and evaluation of the output voltage of theswitched-mode power supply

through measurement and evaluation of a voltage on the secondary side ofthe voltage transformer, for example the alternating voltage between thetransformer and the rectifier and filter.

This monitoring ensures that load variations take place only during theactive phase of the voltage transformer, such that the output voltagedoes not fall below a minimum value. This could otherwise occur when theoutput load is switched in at the start of the burst cycle.

An active phase in high output-load burst operation is present forexample when the ratio of the variation in output voltage Uo to thevariation in time t is positive, i.e.$\frac{\mathbb{d}U_{O}}{\mathbb{d}t} \geq 0.$

An example of direct monitoring is measurement of the voltage betweenthe transformer and rectifier/filter assemblies. If an alternatingvoltage is recorded here, the switching transformer is active and loadsmay be switched in.

In another variant, the output load may additionally comprise a devicefor transmitting a load variation which occurs to the open-/closed-loopcontrol device. It may thus be ensured that any burst mode which may beactivated is deactivated prior to or during the load variation.

Other combinations of the stated switch-on conditions are likewisepossible.

One advantage of the invention is control of the intermediate circuitvoltage in integrated preconditioners without additional activecomponents.

Another advantage of the solution according to the invention is that, inthe case of the above-described method, an energy-saving burst mode isgenerated.

Another advantage of the method according to the invention is theautomatic change-over between the normal mode and burst mode operatingmodes, which change-over is performed by the closed-loop control.

An additional advantage of the method according to the invention is thegreater degree of freedom, in that the control curves of the rectifierand the voltage transformer have to be congruent only to a relativelysmall extent.

Advantageously, load steps on the output side of the switched-mode powersupply are synchronized, which prevents possible problems upontransition from burst mode to normal mode. Depending on circuit design,it may otherwise happen that the output voltage falls below a minimumvalue when the load is switched in at the start of a burst cycle. Thefollowing options are therefore provided:

a) open-loop control of the load, which allows switching in of arelatively large load only during the active phase of the voltagetransformer or

b) open-loop control of the voltage transformer by means of anadditional control line, which terminates the burst mode when the loadis switched in.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted, wherein

FIG. 1 is a block diagram of an example of embodiment of theswitched-mode power supply according to the invention;

FIG. 2 shows a detailed embodiment of the invention and

FIG. 3 shows in Subfigs. a) to d) an example of a burst mode profile.

FIG. 1 is a block diagram of an example of embodiment of theswitched-mode power supply according to the invention. The switched-modepower supply 1 comprises at least two input terminals 2′ and 2″ and atleast two output terminals 3′ and 3″. The input lines are connected forexample to the power mains 4, which has different values in differentcountries, but typically provides between 100 and 240 volts ofalternating voltage. A power consumer 5 is connected as load to theoutput lines 3′ and 3″ of the switched-mode power supply 1. The outputvoltage Uo or the output voltages Uo(l . . .n) is/are so dimensioned asto match the applications thereof.

The switched-mode power supply 1 comprises firstly a preconditioner 6,which rectifies (DC) the input-side alternating voltage (AC). Thevoltage transformer 7 connected thereto converts the direct voltage (DC)into a desired direct voltage (DC). The switching means 13 of thevoltage transformer 7 are also used at the same time for operation ofthe preconditioner. The voltage transformer 7 may take the form, forexample, of a DC/DC converter, which only intermittently accepts thedirect voltage by means of a clocked switching means 13. Thus, theoutput voltage Uo of the switched-mode power supply 1 may be selectedwithin given limits. Between the preconditioner 6 and the voltagetransformer 7, the intermediate circuit voltage Uic is tapped at avoltage divider, which comprises for example 2 resistors R1, R2, andsupplied via a connecting line 19 to the open-/closed-loop controldevice 18. The assembly 9 is used inter alia to measure and monitor theoutput voltage Uo. The assembly 9 has access to the secondary side ofthe voltage transformer 7 via the connection 10. The open-/closed-loopcontrol device 18 keeps the output voltage Uo monitored by the assembly9 and/or the output current constant at least within given limits, byactivating the switching means 13 appropriately via the connection 20.

The open-/closed-loop control device 18 may have a connection 22 to theinput of the preconditioner 6, for the purpose of optional input voltagedetection. Detection of the input voltage allows purposeful switching onof the voltage transformer 7 as a function of the input line voltage,for example at zero transition.

FIG. 2 is a detailed representation of an example of embodiment of theinvention. In the rectifier 11, first of all an unsmoothed directvoltage Udirect is generated from the power mains. Together with thecapacitor C1, the integrated preconditioner functional unit 12transforms this into an approximately constant intermediate circuitvoltage Uic, wherein a current Iline corresponding to the appropriatestandards is at the same time drawn from the power mains by thepreconditioner functional unit 12. From the intermediate circuit voltageUic, the switching stage, which comprises the switching means 13 and theopen-/closed-loop control device 18, indirectly generates the outputvoltage Uo. The assembly 9 measures the output voltage Uo and transmitsit to the open-/closed-loop control device 18, which additionallymeasures the intermediate circuit voltage Uic. In the example ofembodiment illustrated, the output voltage Uo across the output lines 3′and 3″ is potentially isolated. Potential isolation is effected forexample by means of a transformer 14, which transforms the intermediatecircuit voltage Uic to the desired voltage Unorm. The intermediatecircuit voltage Uic is converted into an alternating voltage Ualtematingfor the purpose of transformation by the switching stage, while therectifier and filter 15 convert the alternating voltage into the desireddirect voltage. The rectified output signal Uo is transmitted forexample by an optocoupler 17 and measured and monitored by the assembly9. The coupling 16 between the preconditioner functional unit 12, theswitching stage and the transformer 14 ensures that, in normaloperation, an acceptable intermediate circuit voltage Uic and an inputcurrent waveform meeting the appropriate standard are achievedsimultaneously in the event of closed-loop control of the output voltageUo. This coupling 16, which is merely suggested in the Figure, may beachieved for example by capacitors, inductors and/or magneticcomponents. The inductor of the preconditioner 6 may be integrated intothe transformer 14 for example, but other configurations are alsopossible. Since, in the case of conventional switched-mode powersupplies, only the output voltage Uo is susceptible to closed-loopcontrol, the other two conditions, namely input current Iin andintermediate circuit voltage Uic, have to be achieved by appropriatecircuit dimensioning. This leads to extreme limitations in design andhas proven impossible in the case of low-load operation. According tothe invention, the intermediate circuit voltage Uic is therefore tappedvia the voltage divider RI, R2. The output voltage Uo is measured bymeans of the resistor R3 and the optocoupler 17 and transmitted to theopen-/closed-loop control device in the switching stage. The assembly 9,serving as a measuring device, is connected for this purpose to theopen-/closed-loop control device 18 of the switching stage via the lines8′ and 8″. While the intermediate circuit voltage Uic is below the limitvalue Ulimit, the voltage transformer 7 is switched in such a way by theopen-/closed-loop control device 18 in the switching stage that theoutput voltage Uo is constantly at its desired value Unorm. Only whenthe limit value Ulimit of the intermediate circuit voltage Uic isexceeded is the voltage transformer 7 switched off briefly, i.e. a burstcycle is started. During this interval, measurement of the outputvoltage Uo serves in determination of the time at which switching backon should occur. When the output voltage falls below a minimum tolerablelevel (Umin), the voltage transformer 7 is switched on again. When theintermediate circuit voltage Uic reaches the limit value Ulimit, thenext burst cycle is triggered.

FIG. 3 shows, in Subfig. a) under what load the switched-mode powersupply 1 is operated, in Subfig. b) the on or off status of the voltagetransformer 7, in Subfig. c) the profile of the intermediate circuitvoltage Uic and the recorded limit value Ulimit and in Subfig. d) theprofile of the output voltage Uo and the recorded minimum value Umin.For the period t<t1, operation of the switched-mode power supply 1 isshown as under normal load, which means that the voltage transformer 7is switched on, the intermediate circuit voltage Uic is below the limitvalue Ulimit and the output voltage Uo is regulated to its rated valueUnorm. At the time t1 in the example, low-load operation starts.Initially, the voltage transformer 7 remains switched on, theintermediate circuit voltage Uic increases and at the time t2 reachesthe limit value Ulimit. Thereupon, the voltage transformer 7 is switchedoff, whereby the increase in the intermediate circuit voltage Uic isstopped and the value of the output voltage Uo falls. By the time t3,the output voltage Uo has reached the tolerable minimum value Umin. Toprevent the voltage from falling below this value, the voltagetransformer 7 is switched on again. By the time t4, the output voltageUo has reached a virtually constant value, whereafter the intermediatecircuit voltage Uic increases again, however, since the switched-modepower supply 1 continues to operate under a low load. By the time t5,the intermediate circuit voltage Uic has again reached its limit valueUlimit and the next burst cycle starts. This is repeated until theintermediate circuit voltage Uic again reaches its limit value Ulimit,i.e. the switched-mode power supply 1 operates under a low load. In theexample illustrated, low-load operation ends at the time t6 andconsequently the intermediate circuit voltage Uic no longer reaches itslimit value Ulimit, the output voltage Uo remains above a minimum valueUmin and the voltage transformer 7 is accordingly switched on. Burstmode begins with the first switch-off of the voltage transformer 7 andends when the voltage transformer 7 remains switched on. Burst modecomprises one or more burst cycles, wherein a burst cycle likewisebegins when the voltage transformer 7 is switched off, and includesreaching of the limit value Ulimit of the intermediate circuit voltageUic and of the minimum value Umin of the output voltage Uo, whereinmeanwhile the voltage transformer 7 is switched back on. The burst cycleends or the next burst cycle begins when the voltage transformer 7 isswitched off again. The open-/closed-loop control device 18 monitors anddetermines the intermediate circuit voltage Uic and the output voltageUo. The voltage transformer 7 is switched on or off as a function of thevalues thereof.

In a further embodiment of the invention, the output voltage does notdetermine the time at which switching back on occurs, but t₃ −t₂ iseither predetermined, adjustable by the user of the switched-mode powersupply or adaptive.

To summarize, the efficiency of the switched-mode power supply 1 inlow-load operation is improved by open-/closed-loop control of theintermediate circuit voltage Uic. The intermediate circuit voltage Uicis activated by activation of a burst cycle. Preconditioner 6 andvoltage transformer 7 share a switching stage, whereby costs are keptdown and integratability is increased.

1. A method of operating a switched-mode power supply, in particular aswitched-mode power supply (1) comprising at least one preconditionerfunctional unit (12), a switching stage (13) with an open-and/orclosed-loop control device (18) and a voltage divider (R1, R2), which isarranged between the preconditioner functional unit (12) and theswitching stage, in which an intermediate circuit voltage (Uic) ispresent between the preconditioner functional unit (12) and theswitching stage, characterized in that a limit value (Ulimit) is fixedfor the intermediate circuit voltage (Uic).
 2. A method as claimed inclaim 1, characterized in that the open- and/or closed-loop controldevice 18 activates a burst cycle in the switching stage when the limitvalue (Ulimit) is reached for the intermediate circuit voltage (Uic) onthe primary side of the voltage transformer (7).
 3. A method as claimedin claim 2, characterized in that the voltage transformer (7) isswitched off when the burst cycle is activated.
 4. A method as claimedin claim 3, characterized in that the burst cycle is deactivated once apredetermined, adjustable or adaptive period has elapsed, which periodis measured from the time of activation of the burst mode onwards, andthe voltage transformer (7) is switched back on.
 5. A method as claimedin claim 1, characterized in that the input line voltage is monitoredand, when the limit value of the input line voltage is reached, theopen- and/or closed-loop control device 18 in the switching stagedeactivates the burst mode.
 6. A method as claimed claim 1,characterized in that the output voltage (Uo) of the switched-mode powersupply (1) is monitored on the secondary side of the voltage transformer(7), a minimum value (Umin) is fixed for the output voltage (Uo) and theburst cycle is deactivated when the minimum value Umin of the outputvoltage Uo is reached and the voltage transformer 7 is switched onagain.
 7. A method as claimed in claim 6, characterized in that the oneopen- and/or closed-loop control device (18) in the switching stagemonitors and effects open- and/or closed-loop control of both theintermediate circuit voltage (Uic) and the output voltage (Uo).
 8. Amethod as claimed in claim 1, characterized in that the preconditionerfunctional unit (12) and the switching stage are integrated.
 9. A methodas claimed in claim 1, characterized in that the voltage transformer (7)is a DC/DC converter, which comprises at least one transistor, which isactivated intermittently.
 10. A method as claimed in claim 1,characterized in that the closed-loop control does not activate burstmode when a high output load is present at the same time.
 11. A methodas claimed in claim 1, characterized in that an output load (5) isconnected, which comprises a device for direct or indirect monitoring ofthe activity of the switching stage (13) and effects load variationsonly during active phases.
 12. A method as claimed in claim 11,characterized in that detection of the active phases of the switchingstage (13) is effected by measurement and evaluation of the outputvoltage (Uo) of the switched-mode power supply (1).
 13. A method asclaimed in claim 11, characterized in that detection of the activephases of the switching stage (13) is effected by measurement andevaluation of a voltage on the secondary side of the voltage transformer(7).
 14. A method as claimed in claim 1, characterized in that an outputload (5) is connected, which comprises a device for transmitting anoccurring load variation to the open- and/or closed-loop control device(18).
 15. A switched-mode power supply (1) having a preconditioner (6)and a voltage transformer (7), in which an intermediate circuit voltage(Uic) is measured, in particular a switched-mode power supply (1) whichis operated by a method described in claim 1, characterized in thatmeans are provided for monitoring the intermediate circuit voltage (Uic)comparing the intermediate circuit voltage (Uic) with a fixed limitvalue (Ulimit) activating a burst cycle when the limit value (Ulimit) isreached on the primary side of the voltage transformer (7).
 16. Aswitched-mode power supply (1) as claimed in claim 15, characterized inthat the intermediate circuit voltage (Uic) and the open- and/orclosed-loop control device (18) of the switching stage have the sameground potential.
 17. Use of a switched-mode power supply (1) as claimedin claim 15, to achieve improved efficiency in the case of low-loadoperation of the switched-mode power supply (1).